Chlorellestadite

Chemical formula: Ca<sub>5</sub>(SiO<sub>4</sub>)<sub>1.5</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>1.5</sub>Cl

Chlorellestadite is a rare mineral of the ellestadite group, a calcium silicate-sulfate with chlorine, occurring as hexagonal crystals or granular aggregates.

## Characteristics Chlorellestadite is a mineral belonging to the ellestadite group within the apatite supergroup. It forms a solid solution with fluorellestadite and hydroxylellestadite, where chlorine is replaced by fluorine or a hydroxyl group, respectively. Typical specimens occur as hexagonal, prismatic crystals, often small in size. It also occurs as granular aggregates or as massive, amorphous fillings. ## Physical Properties This mineral is characterized by a Mohs hardness of 4.5, placing it in the medium hardness group. It has a vitreous luster, sometimes transitioning to resinous. It is transparent to translucent. Its density ranges from 3.03-3.07 g/cm³. ## Colors and Varieties Chlorellestadite is most commonly colorless, white, or gray. Specimens with delicate hues, such as pale yellow, light pink, or pale blue-green, are also found. As a member of the ellestadite group, its chemical composition is variable, but no distinct color varieties with commercial names are recognized. ## History and Name The mineral was first described in 1937 by Duncan McConnell. Its name refers to the dominant chlorine in its composition ("chlor") and its belonging to the ellestadite group. The name "ellestadite" in turn honors A. E. Ellestad, a chemist from the University of Minnesota laboratory, who performed the first analyses of these minerals. ## Uses Chlorellestadite has no industrial applications. It is solely an object of scientific interest and is valued by collectors of rare minerals.

Properties

Mohs hardness
4.5
Luster
Vitreous to resinous
Streak
White
Density
3.03-3.07
Cleavage
Indistinct on {0001} and {1010}
Fracture
Conchoidal to uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification A characteristic feature of chlorellestadite, if visible, is its hexagonal crystal form. Its occurrence in contact-metamorphic rock environments (skarns) is a strong diagnostic indicator. Vitreous to resinous luster and moderate hardness also aid in identification. ## Distinguishing from Similar Minerals Chlorellestadite is difficult to distinguish from other minerals of the ellestadite group (fluorellestadite, hydroxylellestadite) without advanced chemical (EDS) or X-ray (XRD) analyses. It can be confused with apatite, which is, however, harder (5 on the Mohs scale). From other white skarn minerals, such as wollastonite or some feldspars, it is distinguished by its crystal form and specific parageneses. ## Crystal Forms Chlorellestadite crystals have a hexagonal habit, ranging from short, tabular to elongated, prismatic. They often form radial or granular aggregates. They also occur as anhedral masses (not exhibiting external crystallographic forms).

Geological environment

## Genesis Chlorellestadite is a typical metamorphic rock mineral, forming under high-temperature conditions. Its main environment of formation is skarns, which are rocks formed at the contact of magmatic intrusions with calcium carbonate-rich sedimentary rocks, such as limestones and marls. It is less commonly found as a product of volcanic exhalations (fumaroles). ## Mineral Associations This mineral often co-occurs with other minerals typical of skarns, such as calcite, diopside, wollastonite, vesuvianite, monticellite, spurrite, and other calcium silicates. ## Localities The most important and classic chlorellestadite localities worldwide include: - Crestmore, Riverside County, California, USA (type locality). - Chichibu Mine, Saitama Prefecture, Japan. - Eifel volcanic complex in Germany. - Vesuvius and Monte Somma volcanic complex in Italy. - Latium region in Italy.

Rarity

Rare

For collectors

## Quality Criteria The most sought-after specimens by collectors are those with well-formed, sharp, and undamaged crystals with a hexagonal prismatic habit. Transparency and clear color are highly valued, although even opaque crystals of good shape are valuable. The attractiveness of a specimen is significantly enhanced by its placement on a contrasting rock matrix, as well as the presence of rare associated minerals. ## Popular Localities Specimens from the type locality in Crestmore, California, are considered classic and most representative for this mineral. Crystals from the Japanese Chichibu Mine are also very popular, often occurring in association with other skarn minerals.

Care and storage

## Cleaning Chlorellestadite specimens should be cleaned very carefully. The safest method is to use a soft brush to remove dust. If necessary, the specimen can be rinsed in distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can cause cracks. ## What to Avoid The mineral is sensitive to acids, which can damage it. Due to its hardness of 4.5, it is susceptible to scratching by harder minerals (e.g., quartz, beryl). Impacts and sudden temperature changes should be avoided. It is stable in sunlight. ## Storage Collector's specimens of chlorellestadite should be stored in separate boxes or display cases, away from minerals of greater hardness. Providing protection against dust and mechanical damage is crucial for preserving their value.

Sources

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